[Go] Generate iterator for vectors

[chriscraws]

Go 1.23 introduced support for iterators. This allows code to define functions that supply values for Go's for range syntax.

Adding an optional flag for generating iterators would be a non-breaking change that allows for a more ergonomic way to visit all items in a vector.

Code such as

var weapon sample.Weapon
for i := 0; i < monster.WeaponsLength(); i++ {
     monster.Weapons(&weapon, i)
     ...
}

could be instead written as

for i, weapon := range monster.WeaponsIter() {
    ...
}

This would be accomplished via generated code that appears as follows (or similar):

func (rcv *Monster) WeaponsIter() iter.Seq2 {
    var weapon Weapon
    return func(yield func(i int, v *Weapon)) {
        for i := 0; i < rcv.WeaponsLength(); i++ {
            rcv.Weapon(i, &weapon)
            yield(i, &weapon)
        }
    }
}

I can author a PR if this feature would be acceptable.